Swelling of Steel Film by Hydrogen Absorption at Cathodic Potential in Electrolyte

Abstract

An ≈4 nm FeCrNi film, deposited on a Ru/B4C multilayer (ML), is used to study cathodic hydrogen charging in electrolyte. A thin film on a ML allows obtaining precise quantitative information on surface metal composition and oxidation state using the X‐ray standing wave technique combined with near‐edge X‐ray absorption spectroscopy. The metal composition is found being close to the composition of stainless steel (SS) 304, and, as for bulk steel, the outer 2 nm passive layer, consisting of oxidized iron and chromium, is depleted of nickel. Overall, it is found that the film represented a useful replica of the surface of bulk steel. Following exposure to 0.1 m KCl electrolyte at −0.6 V versus Ag/AgCl, 11.3 (±3)% swelling of the film by hydrogen absorption is observed. The estimated absorbed amount is exceeding reported bulk absorption under similar conditions by more than an order of magnitude. Strong hydrogen absorption appears to be enabled by the 2D character of the thin film, i.e., a significantly lower associated strain energy compared with bulk absorption. The strong surface swelling is suggested to be related to the lowering of the pitting corrosion resistance of SS surfaces reported following hydrogen exposure.

Structure of the Surface Region of Stainless Steel: Bulk and Thin Films

“…[23] Thus, the thin films appear to be reflecting the behavior of the investigated bulk steel surface region. This confirms our earlier conclusions that steel thin films represent valuable replicas of the surface region of stainless steel [23,26] despite the fact that the "production" histories of the bulk steel samples and the thin films are obviously quite different. Analysis of the XANES data of the 50 nm film reveals that 30% of the iron is adopting the ferrite structure, which can be compared with the GIXRPD results on the polished steel sample.…”

“…We recently reported that cathodic hydrogen loading is stressing the steel surface immensely. [26] However, with concentrations in the ppm range, it is questionable whether the hydrogen concentration in steel following hydrogen in-diffusion under ambient conditions [41,42] could introduce sufficient strain.…”

“…[23] Copyright 2021, The Authors, published by IOP Publishing Ltd.). [26] lab source (X-ray energy of 8.04 keV). With an angle of incidence of roughly 22 for the fcc (111) and bcc (110) peak, the mean probing depth of the X-rays can be calculated to %1.5 μm, significantly larger than for the GIXRPD measurements shown in Figure 3.…”

“…[29][30][31][32] The results revealed that the metal oxidation is limited to the outer 2-3 nm surface region of the SS 304 film. [23,26] The depth distribution of Fe, Cr, and Ni, the three main elements of a 4 nm SS 304 steel in the film, was also analyzed with the help of X-ray standing wave (XSW) measurements. The XSW analysis of the 4 nm SS 304 film on a Ru/B 4 C multilayer has been published recently.…”

“…An important part of the investigation involved the characterization of the thin films using X‐ray diffraction and spectroscopy, which proved that they represent excellent replicas of the (near‐)surface region of steel in terms of structure, chemistry, and composition. The results of X‐ray absorption near edge structure (XANES) [ 25 ] measurements, obtained for an about 4 nm SS 304 film have been published recently [ 23,26 ] and are reproduced in Figure 1 . Analysis of the chemical sensitive XANES spectra revealed quantitatively the degree of oxidation of the three main metallic elements, i.e., iron, chromium, and Nickel.…”